This invention relates to a molded transformer whose windings are cast-insulated with a cast resin.
The insulating structure of the windings of a prior-art molded transformer has been shown in FIG. 1. This figure is a partial vertical sectional view showing only the one-side half of the windings disposed on a core. Referring to the figure, numeral 1 designates the core, numeral 2 a high-voltage coil which is wound round the core 1, numeral 3 a resin insulation layer which cast-insulates the high-voltage coil 2, numeral 4 a low-voltage coil which is wound round the core 1 inside the high-voltage coil 2, numeral 5 a cushion material, numeral 6 a coil supporting spacer which is disposed in order to secure an air gap 7 necessary for insulation between the high-voltage coil 2 and the low-voltage coil 4, and numeral 8 a winding supporting member which holds the coils 2 and 4 through the coil supporting spacer 6 and whose end remote from the spacer 6 is fixed to the core 1.
The insulating structure of the windings of the prior-art molded transformer is based on insulation with the resin insulation layer 3 and the air gap 7 or the coil supporting spacer 6 combined. More specifically, the insulation between the high-volatage coil 2 and the low-voltage coil 4 is the combined insulation utilizing the part of the resin insulation layer 3 inside the high-voltage coil, the air gap 7 between the high-and low-voltage coils and the part of a resin layer (not shown) outside the low-voltage coil. The insulation between the high-voltage coil 2 and the winding supporting member 8 is also the combined insulaton utilizing the resin insulation layer 3 of the high-voltage coil and the coil supporting spacer 6.
In this case, the dielectric breakdown is determined by the electric field intensity of the air gap 7 in the structure of the combined insulation of the resin insulation layer 3 and the air gap 7, and the high breakdown voltage of the resin insulation layer 3 having an excellent dielectric strength is not fully exploited. Accordingly, the air gap between the high-and low-voltage coils must be formed larger than dimensions required for cooling. In addition, the coil supporting spacer needs to be arranged. These lead to the disadvantage that the dimensions of the molded transformer and the whole weight thereof increase.